1. Field of the Invention
This invention relates to a liquid injection recording apparatus which discharges liquid droplets and effects recording by the discharged liquid droplets.
2. Related Background Art
To cause recording liquid droplets to be discharged from a liquid injection head, means is adopted for surrounding the outer wall of the pressure chamber of the injection head by electromechanical converting means such as a piezo-electric element PZ as shown in FIG. 1 of the accompanying drawings, expanding or contracting the piezo-electric element by a variation in a driving voltage applied to the piezo-electric element and causing recording liquid droplets to be discharged by the sudden contraction of the piezo-electric element after the expansion thereof.
A driving circuit therefor, as known from Japanese Patent Application Laid-Open No. 212274/1984, is designed such that a voltage in a direction opposite to the direction of polarization of the piezo-electric element is first applied to the piezo-electric element to expand the piezo-electric element and then a voltage in the direction of polarization is applied to the piezo-electric element to contract the piezo-electric element and effect discharge of recording liquid. The driving circuit is further designed such that at the next timing, no voltage is applied between the two electrodes of the piezo-electric element and the charge stored in the piezo-electric element is discharged to a resistor R provided in parallel to the piezo-electric element (FIG. 2 of the accompanying drawings). In FIG. 2, however, a switching transistor Tr2 is turned on by a control signal CO2 to expand the piezo-electric element and hold it expanded for a short time, whereafter the transistor Tr2 is turned off and simultaneously therewith, a transistor Tr1 is turned on by a control signal CO1 to thereby bring about sudden contraction of the piezo-electric element, and this has led to the possibility that the through-current from Tr1 to Tr2 is produced due to the delay of the switching time of the transistor Tr2 or the timing shift of the control signal to damage an output transistor (FIG. 3).
Also, the stability of discharge is affected by the attenuation wave form, and since the time constant thereof depends on the irregularity of the piezo-electric element, it is difficult to make the discharge characteristic uniform.
Particularly during the attenuation, the impedance of the driving circuit as viewed from the piezo-electric element is high and therefore, the vibration to which the piezo-electric element is subjected cannot be taken up, and this has led to a problem that the stability of discharge is reduced (FIG. 3).